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The assumption that noninvasive prenatal testing for common chromosomal abnormalities (see Chapter 7) is a screening and not a diagnostic test, is unfortunately common. Many women receiving a normal report opt to avoid an amniocentesis. The vast majority will be vindicated, but some will complete pregnancy with a child having a disorder that could have been diagnosed in early gestation. Physicians and counselors are advised to remind women of this limitation, given that about half of all chromosomal abnormalities will be missed by the noninvasive screen.⁷⁰³

Prospective parents should understand their specific indication for prenatal tests and the limitations of such studies. Frequently, one or both members of a couple fail to appreciate how focused the prenatal diagnostic study will be. Either or both may have the idea that all causes of intellectual disability or congenital defects will be detected or excluded. It is judicious for the physician to urge that both members of a couple come for the consultation before CVS or amniocentesis. Major advantages that flow from this arrangement include a clearer perception by the partner regarding risks and limitations, a more accurate insight into his family history, and an opportunity to detect an obvious (although unreported or undiagnosed) genetic disorder of importance (e.g. Treacher–Collins syndrome, facioscapulohumeral dystrophy or one of the orofacial–digital syndromes). Women making an appointment for genetic counseling should be informed about the importance of having their partner with them for the consultation, avoiding subsequent misunderstanding about risks, options, and limitations.

Before prenatal genetic studies are performed, a couple should understand the inherent limitations both of the laboratory studies and, when relevant, of ultrasound. For detection of chromosomal disorders, they should be aware of potential maternal cell admixture and mosaicism (see Chapter 11). When faced with potential X‐linked hydrocephalus, microcephaly, or other serious X‐linked disorders, and the realization of less than 100 percent certainty of diagnosis, couples may elect fetal sex determination as the basis for their decision to keep or terminate a pregnancy at risk. For some, neither chromosomal microarrays, biochemical assays, nor DNA analyses will provide results with 100 percent certainty.

The time taken to determine the fetal karyotype or other biochemical parameters should be understood before amniocentesis. The known anxiety of this period can be appreciably aggravated by a long, unexpected wait for a result. The need for a second amniocentesis is rarer nowadays but, in some circumstances, fetal blood sampling remains an additional option that may need discussion. Despite the very unlikely eventuality that no result may be obtained because of failed cell culture or contamination, this issue should be mentioned.

The potential possibility for false‐positive or false‐negative results should be carefully discussed when applicable. Any quandary stemming from the results of prenatal studies is best shared immediately with the couple. The role of the physician in these situations is not to cushion unexpected blows or to protect couples from information that may be difficult to interpret. All information available should be communicated, including the inability to accurately interpret the observations made. This is especially so with the use of the chromosomal microarray (see Chapter 13) and whole‐exome sequencing (see Chapter 14). Cautions are appropriate with special reference to VOUS (see Chapter 14), that require in addition, parental samples to determine inherited or de novo changes.

Other key issues to be considered by the genetic counselor and discussed when appropriate with the consultand follow.